(Reuters) - The smaller of twin panda cubs born over the weekend to giant panda Mei Xiang died on Wednesday after experiencing respiratory difficulty, Washington's National Zoo said.
The death of the cub, whose up-and-down weight since the birth on Saturday had raised concerns among the zoo keepers, occurred shortly after 2 p.m. on Wednesday.
The twins' birth captured international attention as giant pandas are among the world's most endangered species.
"We are sad to report that the smaller of the two panda cubs has died," @NationalZoo posted on Twitter.
"The larger cub appears to be strong, robust, behaving normally and is with mother Mei Xiang," the zoo said in a press release.
After the twins were born, zoo keepers decided to swap the cubs in Mei Xiang's possession every four hours so each had time to nurse and spend quality time with her.
"We had some concerns that a cub could be injured or both could be injured," the zoo's chief veterinarian, Don Neiffer, recalled at a news conference on Wednesday.
On Tuesday, however, Mei Xiang would not switch and zoo officials became concerned for the smaller cub. Both baby pandas were still fur-less and about the size of a stick of butter.
The most recent swap took place on Wednesday morning and staff noticed the smaller cub's weight was faltering. It appeared weak and was exhibiting respiratory difficulties, Neiffer said.
"Things turned and the cub's condition declined into the afternoon," he added.
Mei Xiang was artificially inseminated with sperm from Hui Hui, a panda in China, and from the National Zoo's Tian Tian.
Zoo officials have said that they did not yet know which insemination was successful, and that it was possible the twins had different fathers.
Giant pandas, native to China, have a very low reproductive rate, especially in captivity. There are about 300 giant pandas in captivity and roughly 1,600 in the wild.
(Reporting by Barbara Goldberg in New York; Editing by Sandra Maler)